Please find below a few quotes from Jonah Lehrer's great and long post (you should read yourself) on the myth of brainstorming:

"Decades of research have consistently shown that brainstorming groups think of far fewer ideas than the same number of people who work alone and later pool their ideas."

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"2003, Charlan Nemeth, a professor of psychology at the University of California at Berkeley, divided two hundred and sixty-five female undergraduates into teams of five." …

"The results were telling. The brainstorming groups slightly outperformed the groups given no instructions, but teams given the debate condition were the most creative by far. On average, they generated nearly twenty per cent more ideas. And, after the teams disbanded, another interesting result became apparent. Researchers asked each subject individually if she had any more ideas about traffic. The brainstormers and the people given no guidelines produced an average of three additional ideas; the debaters produced seven." …

"According to Nemeth, dissent stimulates new ideas because it encourages us to engage more fully with the work of others and to reassess our viewpoints."

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"Another of her experiments has demonstrated that exposure to unfamiliar perspectives can foster creativity." …

"Criticism allows people to dig below the surface of the imagination and come up with collective ideas that aren’t predictable." …

"… the artists could interact efficiently – they had a familiar structure to fall back on – but they also managed to incorporate some new ideas. They were comfortable with each other, but they weren’t too comfortable."

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"If you want people to work together effectively, these findings reinforce the need to create architectures that support frequent, physical, spontaneous interactions”. … “Even in the era of big science, when researchers spend so much time on the Internet, it’s still so important to create intimate spaces."

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"The fatal misconception behind brainstorming is that there is a particular script we should all follow in group interactions. The lesson of Building 20 is that when the composition of the group is right – enough people with different perspectives running into one another in unpredictable ways – the group dynamic will take care of itself. All these errant discussions add up. In fact, they may even be the most essential part of the creative process."